This invention relates to electric discharge devices and particularly to improved electrodeless discharge fluorescent lamps comprising mercury vapor and radio-frequency (RF) power supplies, wherein the mercury vapor pressure is controlled by means of amalgam-forming metals. As is known, electrodeless discharge lamps have several significant advantages over conventional incandescent lamps, including a far greater efficiency in converting electrical energy into light energy. The commercial development of these lamps has been hindered, however, by several technical problems, one of which is their inability to produce a consistent light output over a wide range of ambient temperatures.
As is well known, the mercury vapor pressure within fluorescent lamps must be maintained within prescribed limits during operation to optimize light output. Typically, a range of pressure from about 4.5 mTorr to about 7.0 mTorr yields an acceptable light output. At higher pressures, ultraviolet (UV) photons produced by excited mercury atoms become self-imprisoned, and are thus prevented from reaching the vessel wall where they can activate the phosphor layer, which converts UV photon energy into visible light. At lower pressures, insufficient mercury is available to generate the UV radiation necessary to the operation of the lamp. Since the mercury vapor pressure is a function of temperature, proper regulation of the mercury vapor pressure is difficult to achieve, as the temperature of the lamp is subject to change due to internally generated heat and changes in ambient temperature.
A degree of control over mercury vapor pressure can be obtained by placing a predetermined amount of an amalgamating metal within the lamp at a location where it will combine with the mercury and form an amalgam that operates at a preselected temperature when the lamp is energized. Since the mercury vapor pressure surrounding such an amalgam is lower than the vapor pressure of pure mercury at the same temperature, the amalgam retains control of the mercury vapor pressure.
While the presence of the amalgam reduces the mercury vapor pressure in a hot lamp, it also unfortunately reduces the mercury vapor pressure when the lamp is cool. Thus, lamp efficiency is decreased until the amalgam is brought up to operating temperature.
In addition to the efficiency problems normally associated with fluorescent lamps, RF-excited electrodeless fluorescent lamps present special problems with respect to temperature control. First, since they are intended to replace incandescent lamps in a wide range of applications, they must operate over a broader range of ambient temperatures than is normally expected of fluorescent lamps. Second, before a plasma forms to create a reflected impedance, the RF power supply is unloaded. Therefore, these lamps must turn on quickly to avoid damage associated with extended overdriving of the power supply. Finally, RF-excited light sources tend to have higher energy densities in the plasma discharge and higher wall loadings than conventional tube-type fluorescent lamps, which are characteristics that make control of the amalgam operating temperature more difficult.
Prior electrodeless discharge lamps address these problems by placing some amalgam material at a location within the lamp where it can be heated quickly by the discharge formed in the lamp when the lamp is energized. Energy transferred from the plasma heats the start-up amalgam and quickly brings it to the desired temperature. Under steady-state conditions, the start-up amalgam in the plasma is too hot for optimum lamp efficiency, so mercury vapor pressure control automatically transfers to a control amalgam at a cooler location in the lamp. Such a system is described in U.S. Pat. No. 4,622,495, entitled "Electrodeless Discharge Lamp With Rapid Light Build-Up."
In such a two-amalgam system, the control amalgam is in thermal contact with a relatively cool spot on an inside surface of the lamp, and the amalgamating metal of the control amalgam is selected to obtain the proper mercury vapor pressure at a desired operating temperature. Because the control amalgam is in thermal contact with the bulb of the lamp, the temperature of the bulb essentially controls the mercury vapor pressure. Since the bulb temperature is dependent on ambient temperature, the mercury vapor pressure determined by the control amalgam is sensitive to changes in ambient temperature. This sensitivity to changes in ambient temperature decreases the range of ambient temperatures over which the lamp effectively operates.
Another problem of prior art lamps results from the use of a relatively low-temperature control amalgam. When the lamp is first switched-on, the start-up amalgam quickly releases its mercury. The much cooler control amalgam will attempt to control the mercury vapor to a lower pressure by collecting mercury from the lamp vessel. The control amalgam will continue to pull mercury from the lamp vessel, and thus reduce the mercury vapor pressure below the optimal level, until the control amalgam reaches the desired operating temperature.
As a result of the control amalgam's tendency to provide unacceptably low mercury vapor pressure during start-up, a low-temperature control amalgam must be selected so that the control amalgam attains its operating temperature quickly. Otherwise, the lamp will suffer significantly decreased light output during warm-up.
While the use of a low-temperature control amalgam gives a reasonable light output during warm-up, there are difficulties associated with keeping low-temperature amalgams cool. For example, since power electronics produce heat, the need for a low operating temperature increases the difficulties associated with integrating electronics into the lamp and limits the amount of power that may be supplied to the lamp. Moreover, prior art lamps rely on lamp surfaces to conduct heat from control amalgams to keep the amalgams cool. Because these lamp surfaces are sensitive to changes in ambient temperature, the control amalgam is also sensitive to changes in ambient temperature. This sensitivity to ambient temperature decreases the range of ambient temperatures over which prior art electrodeless discharge lamps operate efficiently and increases the difficulty of stabilizing the mercury vapor pressure. These constraints seriously limit the general usefulness of these lamps as replacements for conventional incandescent lamps.
For the foregoing reasons, there is a need for fast-starting electrodeless discharge lamps wherein the light output is optimal over a broad range of ambient temperatures.